Japanese Patent Laid-Open No. 2000-156823 discloses an image-pickup apparatus, in which some pixels (focus detection pixels) included in an image-pickup element used in the apparatus are provided with different optical characteristics from those of other pixels to perform focus detection based on outputs from the focus detection pixels.
In the image-pickup apparatus disclosed in Japanese Patent Laid-Open No. 2000-156823, plural focus detection pixels paired with each other are arranged in part of the image-pickup element. FIG. 5 shows one example of a pixel arrangement of the image-pickup element in which the focus detection pixels are arranged in some of the lines of the pixel matrix.
In FIG. 5, reference symbols R, G, and B respectively represent normal image-pickup pixels provided with a red filter, a green filter, and a blue filter. Reference symbols S1 and S2 respectively represent first focus detection pixels and second focus detection pixels which have different optical characteristics from those of the image-pickup pixels.
FIG. 6 shows the structure of a first focus detection pixel S1. In FIG. 6, a microlens 501 is formed on a light-entrance side of the first focus detection pixel. Reference numeral 502 denotes a planar layer forming a flat surface for providing the microlens 501.
Reference numeral 503 denotes a light-shielding layer, which has an aperture decentered to one direction relative to the center O of a photoelectric conversion area 504 of the first focus detection pixel S1.
FIG. 7 shows the structure of a second focus detection pixel S2. In FIG. 7, a microlens 601 is formed on a light-entrance side of the second focus detection pixel. Reference numeral 602 denotes a planar layer forming a flat surface for providing the microlens 601.
Reference numeral 603 denotes a light-shielding layer, which has an aperture decentered relative to the center O of a photoelectric conversion area 604 of the second focus detection pixel S2. The aperture of the light-shielding layer 603 is decentered in a direction opposite to that of the light-shielding layer 503 provided in the first focus detection pixel S1. That is, the light-shielding layers 503 and 603 have their apertures at symmetric positions relative to the optical axis of the microlenses of the first and second focus detection pixels S1 and S2.
With such a structure, viewing an image-pickup optical system from the first focus detection pixel S1 and from the second focus detection pixel S2 is equivalent to symmetrically dividing a pupil of the image-pickup optical system.
In FIG. 5, in the line containing the first focus detection pixels S1 and in the line containing the second focus detection pixels S2, two images are formed which are more approximate to each other as the number of pixels in the image-pickup element increases. When the image-pickup optical system is in an in-focus state relative to an object, outputs (image signals) obtained from the lines respectively containing the first and second focus detection pixels S1 and S2 match with each other.
On the other hand, when the image-pickup optical system is out of focus, a phase difference is generated in the image signals obtained from the lines respectively containing the first and second focus detection pixels S1 and S2. Directions of the phase difference in a front focus state and in a rear focus state are opposite to each other.
FIGS. 8A and 8B show the relationships between the focus state and the phase difference. In these drawings, both focus detection pixels S1 and S2 shown in FIG. 7 are illustrated closer to each other and designated by symbols A and B. The image-pickup pixels are omitted.
The light flux from a specific point on the object is divided into a light flux ΦLa and a light flux ΦLb, the former entering a focus detection pixel A through a divided pupil corresponding to the focus detection pixel A and the latter entering a focus detection pixel B through a divided pupil corresponding to the focus detection pixel B. These light fluxes come from the identical point on the object. Therefore, when the image-pickup optical system is in an in-focus state, they pass through the same microlens and reach one point on the image-pickup element as shown in FIG. 8A. Accordingly, the image signals respectively obtained from the lines containing the first focus detection pixels A (S1) and second focus detection pixels B (S2) match with each other.
On the other hand, as shown in FIG. 8B, when the image-pickup optical system is out of focus by x, the reaching positions of both light fluxes ΦLa and ΦLb are offset from each other by a change in the incident angle of the light fluxes ΦLa and ΦLb onto the microlenses. Therefore, a phase difference is generated between the image signals respectively obtained from the lines containing the first focus detection pixels A (S1) and second focus detection pixels B (S2).
The image-pickup apparatus disclosed in Japanese Patent Laid-Open No. 2000-156823 performs the focus detection with the image-pickup element utilizing the above principle.
However, when obtaining a still image using such an image-pickup element containing the focus detection pixels, pixel data corresponding to the positions of the focus detection pixels are lost. Since the focus detection pixel has a different viewing field from that of the normal image-pickup pixel, using the signal obtained from the focus detection pixel as an image signal for a still image will cause discontinuity between the signal from the focus detection pixel and the signal from peripheral pixels thereof, which makes it impossible to obtain a good image.
To solve such a problem, in the image-pickup apparatus disclosed in Japanese Patent Laid-Open No. 2000-156823, image signals corresponding to signals from the focus detection pixels are interpolated using image signals from peripheral pixels thereof.
In the pixel arrangement of the image-pickup element shown in FIG. 5, interpolating data from the peripheral pixels are inserted into portions of a picked-up image corresponding to the focus detection pixels S1 and S2. In FIG. 5, the R, G, and B pixels for image pickup are aligned in a Beyer arrangement, and some of the G pixels are replaced by the focus detection pixels S1 and S2. As data of the G pixel which is lost because of the presence of the focus detection pixels S1 and S2, synthesized pixel data generated from data of four G pixels located obliquely adjacent to the G pixel is provided.
However, the interpolation of the image signals of the focus detection pixels by using the image signals of the peripheral pixels thereof as disclosed in Japanese Patent Laid-Open No. 2000-156823 may cause a decrease in sharpness in the image obtained by these peripheral pixels as compared to the image obtained by pixels of other areas.
When receiving light forming an object image having a low spatial frequency, the continuity of an image signal from the focus detection pixel may be low relative to image signals from peripheral image-pickup pixels thereof because of the difference in viewing field between the focus detection pixel and the peripheral image-pickup pixels. Therefore, it is preferable to interpolate the image signal at the position of the focus detection pixel based on the image signals from the peripheral image-pickup pixels. In this case, because the spatial frequency of the object image is low, a decrease in sharpness due to the interpolation is hardly prominent.
On the other hand, when receiving light forming an object image having a high spatial frequency, the continuity of the image signal at the position of the focus detection pixel is originally low relative to the image signals from the peripheral image-pickup pixels thereof. Therefore, a decrease in sharpness due to the interpolation becomes prominent. Accordingly, as the number of the focus detection pixels is increased, image areas with the sharpness lowered due to the interpolation are increased, and the quality of the obtained image is lowered.
When phase difference sensors with different viewing fields are provided on the image-pickup element and focus detection is performed based on the phase difference obtained by these phase difference sensors, the sensors are provided with an aperture on its front face for performing pupil division. The phase difference sensors are not provided with a color filter on its light-entrance surface. Therefore, an image signal output from a phase difference sensor has a different signal level from those of the pixels located in the periphery thereof, which makes it impossible to use the image signal output from the phase difference sensor as it is for still image data.
Incidentally, it is generally known that an image signal at a focus position with a large defocus amount contains a small high-frequency component. In contrast, an image signal at a focus position with a small defocus amount contains a high-frequency component that is the largest in the defocus range.